Carbon Dioxide Emissions.
It is generally agreed that carbon dioxide emissions must be brought under control, but technology has not kept pace with policy. Of particular concern are coal-fired electric power plants, which are indispensable providers of electricity. Non-hydro renewables, such as wind and solar, are negligible sources of power, less than half a percent of coal for the U.S. in 2006. There is no economical means for carbon capture and sequestration at coal plants.
Alarm over global climate change has led to plans for a carbon tax projected between $20-200 per ton of carbon dioxide. Owners of existing coal plants may reasonably elect not to do anything to reduce emissions, but instead pay any carbon tax and pass on the cost to utility customers in a rate increase. Amine scrubbing and underground storage, the leading current proposals for carbon capture and sequestration, would be prohibitively expensive, and there is good reason to doubt that they would be reliable. Flue gas from coal plants contains fly ash, a large nitrogen ballast, and NOx and SOx which are acid precursors. The volume of the waste stream is overwhelmingly large.
There must be some economic incentive other than a punitive tax if there is to be a widely adopted remediation program to avert catastrophic global climate change from uncontrolled CO2 emissions. The present invention offers a positive incentive to curb carbon dioxide emissions by turning CO2 from waste to resource.
IGCC Power Plants
Integrated Gasification Combined Cycle (IGCC) power plants convert coal or biomass, by means of a process called gasification, into combustible syngas (a mixture of carbon monoxide (CO) and hydrogen (H2)). The combined cycles are: (1) a Brayton cycle (where exhaust gas from syngas combustion drives a gas turbine), and (2) a Rankine cycle (a steam turbine, where the waste heat from the gas turbine is used for steam). For the same energy output, an IGCC plant needs 10-20% less fuel than a large-scale pulverized coal power plant. IGCC plants also use about 30% less water than coal fired power plants. The area occupied by IGCC plants is much smaller.
Gasification is a process that converts carbonaceous material into syngas. Even high sulfur coal, lignite, plastic, and landfill can be fuel for IGCC. Gasifiers can be operated as air-blown or oxygen-blown. The air-blown gasifier is inferior to the gasifier which uses pure oxygen. The fuel conversion efficiency of the air-blown gasifier is only 46% vs. 79%, and the energy density, or heating value, of the syngas produced is only 5.3 MJ/kg vs. 12.55 MJ/kg. The oxygen for oxygen-blown gasification is conventionally extracted from the air by cryogenic distillation, a large energy drain.
Along with syngas, the gasification process produces carbon dioxide (CO2). Capturing carbon dioxide after gasification in an oxygen-blown gasifier is easier than post-combustion flue gas carbon capture or pre-combustion capture from an air-blown gasifier because of the absence of nitrogen ballast. Air is 78% nitrogen (N2), and this inert fraction in air or flue gas is called nitrogen ballast. Exhaust gas from syngas combustion to run the gas turbine also contains CO2, which must be captured as well. Amine scrubbing is one method for carbon capture, and chilled ammonia is another.
Carbon Sequestration.
Once carbon dioxide has been captured, something must be done with it. The follow-on to carbon capture is called sequestration. As envisioned presently by policymakers, sequestration is a concealed dumping scheme. The object is to store the carbon dioxide underground or in the ocean instead of in the atmosphere. The enormous volume and weight that must be transported and injected, and the lack of any assurance that the carbon dump will remain secure, should give preference to some sort of treatment at the plant instead of dumping, but presently no carbon dioxide treatment is feasible for the large volumes of hot and dirty waste gas emitted by utilities and industries.
If all of the carbon dioxide emitted by one average 250-MW coal-fired electric power plant in a year were captured, there would be 1.7 million metric tons to dispose of. The density of carbon dioxide gas is 1.98 kg/m3 at standard temperature and pressure so each metric ton (1000 kg) of CO2 at sea level pressure on a warm day occupies 554 cubic meters, about the size of a house. Each year, the carbon dioxide waste stream from this average coal plant would fill a cubic kilometer.
Large coal plants emit as much as 6 million tons per year. Cement plants and refineries and steel mills are also heavy polluters. For example, the Shell Martinez Refinery in the San Francisco Bay Area dumps over 4.4 million metric tons of CO2 into the atmosphere each year. The total CO2 load from the U.S. was over 6 billion metric tons in 2005. That's over 3 trillion cubic meters, or 117 trillion cubic feet. Transporting that much weight and putting that much volume underground every year would be an expensive undertaking.
Buried carbon dioxide gas may percolate back to the surface and leak out to harm people or at least escape into the atmosphere. The experience with sequestering nuclear waste in the United States should be instructive as to the political feasibility of any sequestration scheme. Nuclear waste is still without a site for permanent sequestration, and its volume is minuscule compared to the volume of carbon dioxide waste from only one plant. The citizens of Nevada have firmly declined the honor of hosting a nuclear waste dump at Yucca Mountain, and the same reaction can be expected elsewhere for carbon dumps.
If the pressure is increased to cram more carbon dioxide into available dump space, the danger of leaks, migrations, and eruptions increases. When the likelihood of human error, dishonesty, and greed—as well as earthquakes and other natural disasters—are considered as well, there no reason to expect that public approval can be obtained for siting carbon dumps.
In summary, sequestration is not only prohibitively expensive but also not feasible as a long term solution. A way must be found to transform CO2 into harmless materials. Best of all would be a way to transform carbon dioxide into something useful, like syngas or carbon nanotubes (tubular fullerenes). That is an object of the present invention.
Syngas Synthesis by Simultaneous Electrolysis of CO2 and Water.
Syngas is a mixture of carbon monoxide (CO) and hydrogen (H2) which can be burned directly or used as a feedstock to make synthetic fuel, lubricants, or plastics using the well-known Fischer-Tropsch synthesis process. Carbon dioxide cracking to syngas could provide means for carbon recycling at power plants, using the energy density (heating value) of syngas thus recovered for direct combustion to help power the gas turbine. Or the syngas could be synthesized to make vehicle fuel. The energy density of syngas ranges from 5 to 12 MJ/kg depending on the process used in gasification (oxygen-blown yielding the highest energy density, for superior combustion). For purposes of comparison, the energy density of natural gas is 45 MJ/kg; gasoline is 46.9 MJ/kg, or 34.6 MJ/1 (131 MJ/gallon); lignite is 14-19 MJ/kg; and wood is 6-17 MJ/kg.
The recovered energy from carbon recycling might justify carbon dioxide treatment economically, should taxes and scolding prove ineffective in motivating power producers and others to reduce CO2 emissions. The energy density (heating value) of bituminous coal is 24 MJ/kg. Therefore, recovered syngas at 12 MJ/kg would recover half the energy of coal. The mitigation of the coal cost by carbon recycling would offset some of the cost of carbon dioxide treatment. Moreover, carbon dioxide treatment would avoid the prohibitive costs and other problems of carbon sequestration. Not only coal plants but also natural gas plants could use carbon recycling to offset the cost of carbon dioxide treatment.
Oxygen, as well as carbon, could be recycled if carbon dioxide cracking were available. Pure oxygen is preferable to air in the gasification process because it avoids the nitrogen ballast problem and produces syngas having a higher energy density (approximately 12 MJ/kg). For IGCC, an air separator is used to produce oxygen for oxygen-blown gasification. The air separator accounts for approximately 30% of the operation and maintenance cost of the plant. The mitigation of the air separator cost by means of oxygen recycling would offset the cost of carbon dioxide treatment.
The Idaho National Laboratory has developed a process, which they have dubbed “syntrolysis,” for syngas synthesis by means of simultaneous electrolysis of carbon dioxide and steam at high temperature (830° C.) in a static cell of exotic metals and ceramic materials. The cell is not only expensive, but small. For high volumes of carbon dioxide, the scalability of syntrolysis according to this setup remains unclear.
An object of the present invention is to provide a carbon dioxide cracker which will be able to continuously process voluminous CO2 waste streams into valuable products, such as syngas and nanostructures such as carbon nanotubes.
Electrolytic Dissociation of Waste Gases.
The required energy for molecular dissociation, also known as cracking, can be transferred in many forms, including heat, mechanical, or electrical energy. Electrical energy can be transferred in an arc discharge, as in the case of lightning transforming oxygen into ozone. Normally, gases such as carbon dioxide are nonconductive, but a strong enough electric field dissociates electrons from molecules, a process called ionization, leaving a mix of positively charged ions and free electrons called a plasma. Plasma is a good conductor, so a current flows through the ionized gas in an arc discharge. The arc discharge transfers energy into the gas and increases ionization.
The rate of energy transfer into the gas, by resistive dissipation, is proportional to the square of the current according to the formula P=I2R, where P is power, I is current, and R is resistance to current flow through the gas between the electrodes. Arc discharges which connect the anode and the cathode are undesirable not only because they result in a short circuit of the energy so it does not get dissipated into the gas, but also they cause electrode erosion. The conventional approach to preventing these problems is to interpose a dielectric such as glass between the feed gas and the electrode, as practiced in dielectric barrier discharge (DBD) reactors which are well known in the art of ozonizers. The dielectric barrier has charges distributed evenly over its surface in contact with the feed gas, so there is no local charge concentration, as in a bare metal electrode, which could cause arcing and erosion. The discharge from the dielectric is a multitude of filaments, called a glow or a corona, rather than one concentrated arc. The filamentary currents transfer electrical energy into the gas in a multitude of tiny paths, which is good for resistive dissipation. However, the interposed resistance of the dielectric weakens the E field in the gas between the electrodes, so the electromotive force driving electrical energy into the gas is weak.
Thermal plasma processes, which require high pressures, are impractical for carbon dioxide cracking on an industrial scale. An alternative is the plasma process is called cold or nonisothermal because although electron temperature is thousands of degrees, as in a thermal plasma, the gas temperature is moderate because the gas has not come to thermal equilibrium with the electrons. The fluorescent light is one example. Low (atmospheric) pressure means that the gas molecules excited by electron collisions cannot bump into each other frequently enough to come to thermal equilibrium.
The gliding arc (glidarc) cold plasma reactor operates at approximately atmospheric pressure, and uses transient arcs between the electrodes to transfer energy into the gas for cracking. The weakening resistance of a dielectric barrier is avoided. Instead of a dielectric barrier, the motion of the arc prevents a concentrated arc discharge and thus protects the electrodes from erosion and diffuses electrical energy into the feed gas. Convective cooling of flowing gas between divergent electrodes keeps gas temperature moderate. Glidarc solves the electrode erosion problem by moving the arc along with the gas through which it conducts, thereby moving the arc ends so they do not dwell and erode the electrodes. Glidarc reactors known to the art operate at high voltage with low current. An improved version of glidarc (Glidarc II) comprises one rotating cylindrical electrode nested with a coaxial static electrode, and axial feed flow between the electrodes. A. Czernichowski, et al., U.S. Pat. No. 6,924,608 (2005).
Glidarc reactors have been investigated as means for carbon dioxide cracking to syngas. A. Czernichowski, Oil & Gas Science and Technology—Rev. IFP, Vol. 56, p. 181, pp. 189-196 (2001).
Another improved glidarc reactor, operating at high voltage, incorporates the principle of reverse vortex flow (axial counterflow, as in a cyclone) as practiced in the Ranque-Hilsch vortex tube. Tangentially jetted feed swirls down a tube and then rebounds through a ring cathode at the bottom of the tube up to a disk anode at the top of the tube, in an axial plasma jet. The tube could be held by a bare hand, dramatically demonstrating its cold plasma character. C. S. Kalra, et al., Rev. Sci. Instruments 76, 025110 (2005).
A disadvantage of known glidarc reactors is that residence time of feed gas in the processing zone between the electrodes is short. Feed gas just blows through, which is necessary to move the arc so as to prevent electrode erosion. Improving feed residence time of cold plasma or glidarc reactors is another object of the present invention.
The Glidarc II discussed above and the reactor disclosed by Hayashi, et al., U.S. Pat. No. 5,817,218 (1998) are examples of reactors where there is shear between the electrodes, using shear instead of pressurized gas flow to prevent erosion. Both show a single moving electrode. In the Glidarc II the rotating electrode is cylindrical. Hayashi shows a cold plasma reactor comprising a rotating disk electrode having a layer of catalyst and opposed to a catalyst-coated stationary plate electrode. Feed is peripheral to the turbulent gap between the electrodes. Alternating current at 30-50 kHz is applied to the Hayashi electrodes to create the plasma, although direct current may be used. A reactor for electrolysis comprising an axially fed workspace between co-rotating disk electrodes is disclosed by Fleischmann, et al. U.S. Pat. No. 4,125,439 (1978). Separation of electrolysis products in the Fleischmann, et al. device is by means of an annular splitter disposed between the electrodes.
A need exists for a reactor which has a high rate of energy transfer to the feed, a long residence time of feed between electrodes, minimal electrode erosion, minimal gas blanketing of the electrodes, and good separation of electrolysis products. The present invention addresses that need.
The Disk Dynamo.
Michael Faraday discovered in 1831 that by rotating a copper disk through a space between magnetic poles he could draw off an electrical current from the disk near its axis of rotation. This was the first electrical generator. The homopolar generator, as it became known later, was investigated by Tesla and many others. N. Tesla, “Notes on a Unipolar Dynamo,” The Electrical Engineer, Sep. 2, 1891. Here, it will be referred to as a disk dynamo. The voltage of the disk dynamo may be small (<3 volts), but the current is large (up to millions of amperes). Current applications include welding and rail guns. The potentials used for electrolytic half reactions are within the range attainable with a disk dynamo.
J. Bockris, et al. Int. J. Hydrogen Energy, Vol. 10, p. 179 at 185 (1985) discloses a single disk dynamo rotating through an axial magnetic field. The potential used for water cracking was between the periphery and axis of the same disk.
A charge separation between the disk axis and disk periphery results from the opposite forces on the free positive and negative charges in the disk when the disk rotates through the magnetic field. The motion of the disk is azimuthal and the magnetic, or B field is axial, so the electromotive force (voltage) is radial and opposite for positive and negative charges, causing them to migrate in opposite radial directions. Depending on the direction of rotation and the polarity of the transverse magnetic field, a current flow sets up which may be radially inward, turning the disk periphery into a cathode, or radially outward, turning the disk periphery into an anode.
Carbon Nanotube Synthesis.
Tubular fullerenes, commonly referred to as carbon nanotubes, are a commercially very valuable form of carbon that has many remarkable properties. Carbon nanotubes have 100 times the tensile strength of steel, and may be the strongest material on earth. In their metallic (armchair symmetry) form, they can carry high current with little heat, making them near superconductors. For heat conduction they are the most efficient material known. Each carbon atom connects to three others in a lattice of hexagons rolled into a tube. They have a high degree of resistance to corrosion along their length due to their cohesive molecular structure.
Shortcomings of present carbon nanotube synthesis methods include: amorphous carbon soot and defective structures mixed with the desired structures, small batches, short tube length, kinking, and tangling of produced nanotubes.
Inorganic nanotubes, nanowires, and other fine filamentary structures have also been synthesized. In addition to being made from carbon, nanostructures can also be made from boron nitride, gold, metal dichalcogenides (MX2 (M=Mo, W, Nb, Ta, Hf, Ti, Zr, Re; X=S, Se)), metal oxides, and metal dihalides.
Annealing is recognized to be of benefit in nanotube synthesis Annealing is conventionally understood to be a heat treatment after formation, but mechanical or vibration annealing is also known. The object of annealing is to improve packing of atoms into the desired lattice, to segregate impurities, and to eliminate imperfections in structure. The present invention also improves annealing of nanotubes during synthesis.
A high volume reactor for the continuous synthesis of carbon or other nanotubes is another object of the present invention. If nanotubes could be recovered from CO2 cracking residue, even in low quantities, they might more than offset the expense of reducing carbon dioxide emissions. The incentive to capture and crack carbon dioxide might become profit instead of coercion.